Imágenes de páginas
PDF
EPUB

German Methods.

I’arts I. White potter's clay, free from water. . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Glauber salt, free from water............................ S5 to 100 Coal.----------------------------------------------------- 17 II. White potter's clay, free from water. . . . . . . . . . . . . . . . . . . . . . . . . . . 100 Glauber salt, free from water................................. 41 Soda, free from water............ - - - - - - - - - - - - - - - - - - - - - - - - - - - 1. Sulphur.-------------------------------------------------- 13 Coal.----------------------------------------------------- 17 FRENCh METHOD. White potter's clay, free from water........................... 100 Soda, free from water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . - - - - - - - - - - 100 Sulphur.--------------------------------------------------- 60 Coal.----------------------------------------------------- 11

The next operation to be performed is that of what is called the over-glowing of this mixture. It is placed in melting pots of potter's clay, formed to withstand intense heat, and slowly dried till burnt. Absolute exclusion of air being indispensable, it is especially requisite that the melting pots be so tempered that they will neither burst nor become softened in the intense heat requisite to burn the mass within them. They may vary from 4 to 12 inches in height, with the like diameter. When filled up they are packed one on the other in a furnace resembling in form a i. brickkiln. They occupy the whole centre of the surface, while the space on each side of them is used for the burning of similar pots. The furnace being properly filled, the mouth is walled up, and the firing commences. The burning continues during from seven to eight hours up to three days, according to the size, construction, and contents of the furnace. Fuel must be added till the mass is thoroughly incorporated and begins to melt. Upon this operation everything depends. If it be not properly conducted, the best and most accurately proportioned raw material will not yield a profitable result. The temperature must be of a certain height, which is to be ascertained beforehand by trials in a small testing oven. It approaches a bright red or incipient white heat, and must be kept at the same point during a specified time; and it must be made to heat the whole mass as thoroughly as possible. When the furnace is cooled, the glowed mass is taken out and cooled with water, and then repeatedly washed and drained to remov any salt still remaining. The now dried and spongy mass is next removed to

...the mill and broken and pulverized to the utmost possible degree of fineness;

the powder is repeatedly washed with water, and after being thoroughly dried, again ground and nicely sifted. It has now reached the first stage of ultramarine, or what is called green ultramarine, and is ready either for sale or for transmutation into the blue colored or proper ultramarine. Hitherto, however, the green ultramarine has been in no very great request, as compared with the blue. It varies through several shades, from apple green to blue green; and in beauty it is far excelled by the copper color o even by the cobalt. Its chief, if not its only recommendations are its cheapness and its innoxiousness; and those qualities, important as they undoubtedly are, seem insufficient to counterbalance its want of brilliancy. The next important operation is the transmutation of the green into the blue color. Here there is but one cause for anxiety. To obtain a perfectly beautiful blue, we must previously have a perfectly beautiful green. The latter is roasted with sulphur, air being freely admitted during the process. It sometimes happens that the change of color takes place without any interference. The sulphuric matrium contained in the mass causes spontaneous ignition on the admission of air, and when it ceases to glow we have still sulphuric acid present, and the green color is thus self-changed into a beautiful blue. As to this process also of transmuting the green color into blue the French and Germans have their peculiar methods. The Germans use small iron cylinders for roasting; the French small hearth ovens, into which, however, the flame cannot enter. Hitherto cylinders of potter's clay have not been adopted, though we doubt not that they would serve just as well, and be even more durable. The cylinder being filled with from twenty-five to thirty pounds of green ultramarine, a vane (Flügelwelle) is set in motion so that the contents of the cylinder may not be burnt without being first thoroughly roasted within. A pound of sulphur is now passed through an upper opening into the cylinder, and while the wind-vane continues in motion the sulphur is gradually consumed. The addition of sulphur may be continued as long as the color improves in purity and brilliancy, but care must be taken not to continue it too long. After the color has been thus roasted it must once more be washed, dried, ground, and sifted. The French method of roasting possesses this advantage, that, by allowing a freer accession of air, the green mass is the more speedily transmuted into blue. But, on either the French or German method, a large quantity of sulphuric acid escapes, which renders the factory a nuisance to its neighbors, while, were that quantity of sulphuric acid preserved, it would suffice for the production of all . Glauber salt used in the manufacture. The quality of the green color is the rule and test of that of the blue; but something of its intensity also depends upon the manner in which it is ground— the finer it is powdered the brighter and clearer it becomes. But not all kinds are of equal beauty; lighter tones of color are frequently obtained without there having been any appreciable difference in the mixture of the raw materials. From these lighter and darker shades a medium kind is obtained by mixing them together, and adding other white or light materials. Where this admixture is resorted to, equal tones of color are out of the question; the shades vary from the softest sky blue to a glowing, almost ruddy, dark blue—the former generally forming a more compact powder, the latter a more loose and smooth one. The principle upon which the blue color of the ultramarine is dependent is as undecided now as it was in the time of Gmelin. It is much to be regretted that his analysis of the Lapis lazuli, which so much conduced towards the manufacture of artificial blue ultramarine, has not been repeated and followed up. The foundation which he laid in scientific experiment has been built upon only in the way of the merest empiricism; and the success which has thus, in a merely monetary point of view, been obtained by the manufactures, has led not a few of them to imagine—how vainly we need not say—that henceforth they are quite independent of science. They forget that practical men, however ably they may profit by what science has taught them, do literally nothing towards clearing up what science itself has yet to learn. It was the science of Gmelin which alone laid the foundation for the manufacture of ultramarine as it at present exists; but who shall pretend to limit the improvements that might be made in that manufacture could another Gmelin arise to discover the principle on which the coloring of the ultramarine depends? Attempts have, indeed, often been made to lift the veil from this mystery, but hitherto they have been so made that it was impossible for them to succeed. Analysis has followed analysis, regardless of the fact that the ultramarine trade is not a preparation of determinate composition from which uniform results can be obtained. However accurately the operator may have treated the clay with water and sulphur, does not the color imbibe some portion of silicious matter? Nay, has not each specimen of clay different elements and different proportions of elements in its own composition? IIow are we to tell, even from the most skilful and laborious analysis, which is the essential product and which the accidental Which the portion which conduces to the production of the color, and which the portion that, to a greater or less extent, limits its quantity and diminishes its brilliancy? The time spent in analyses, thus inevitably indecisive, may be considered as completely thrown away. In truth, those analyses have rather raised questions than settled them. The influence of iron, for instance, upon the production and the color, which long ago was considered a settled fact, is now relegated into the realm of doubt. There seems good reason to believe that sulphur has a chief, if not the sole, part in coloring the green and blue ultramarine; but how—through what combinations ! The material itself opposes difficulties to our clear view of the subject; and the difficulties are increased by the coincidence of two chemical processes, and by the facile decomposition of the material the moment it is attacked by reagents. Finally, we are but too imperfectly acquainted with the affinities of sulphur and the recently discovered sulphuric acids for the alkalies. This powerlessness of analysis to pronounce definite judgment has necessarily given rise to various opinions, founded not upon facts, but upon fancies, and, as usual in such cases, the opinion founded upon fancy has been more peremptorily asserted than the knowledge founded upon fact. Of the green ultramarine, we have seen it positively asserted, though without even an attempt at proof, that it is a simple combination of sulphur matrium, while another disputant is not less positive that it is a mixture of blue ultramarine with some yellow substance, the elimination of which turns the green to blue. Others assert that the acid has transformed the sulphur natrium of the green ultramarine into a sulphuric metal, combined with other and unascertained matter; that oxydization has taken place, and the sulphur has united with the undecomposed sulphuric natrium. According to others the oxygen acts upon the sulphur and forms a sub-sulphuric acid, or some other of the recently discovered sulphuric acids. . In short there has been much disputation, but no approach to a conclusion which can be relied upon. To arrive at such a conclusion we must, as our starting point, first study the affinities of aluminum and sulphuric matrium. All that we are thus far warranted in saying is simply this, that ultramarine contains silicious earth, potter's clay, natron, and sulphur. But, what else? That is the real question at issue. The silicious earth is, if not superfluous, at least inoperative, as regards the production of the blue color; but, though not itself the cause of the blue color, it at least supplies the fire-proof quality. Too much of it, undoubtedly, is injurious to the color. If the silicious acid be not fixed by natron, the blue color is either very much faded, or wholly destroyed, and the ultramarine is rendered unfit for the purposes of the porcelain painter. The artificial ultramarine has still another great advantage over the natural. While the latter could only be used for oil paintings, the former can be used in every art in which the blue color is indispensable, and consequently it has, to a very considerable extent, supplanted cobalt, litmus, and Prussian blue. Even when the ultramarine commanded a far higher price than smalt, (the latter selling, in France, at 47 to 50 cents per pound, while the former could not be purchased for less than from $162 to $173,) it was found that ultramarine was the cheaper article, for the simple reason that one pound of ultramarine would do the work of ten pounds of cobalt blue. Ultramarine is a reliable color for oil painting or for painting on glass, for tapestry, and for paper-hangings in patterns, and for the coloring of soaps, candles, &c., &c., and it is not easy to over-estimate its importance in printing on wool, cotton, linen, or silk. To the French manufacturer, Blondin, belongs the credit of having been the first to use ultramarine in cotton printing. For six years he kept his application a secret; but, in 1844, Dolfus, a cotton manufacturer from Alsace, visited the French exhibition and made himself master of the process. Since then, as is said in the report of the French exhibition of 1849, “Guimet's method has travelled round the world, supplanting all the blue colors which had been previously employed by the cotton-printer.” It must be confessed, however, that this statement is not quite exact, for the manufacturers still experienced some difficulty in using ultramarine. At first the color was, for the most part, not sufficiently fine, and consequently it affected both the spreading-knife and the rollers. That difficulty was obviated by the use of albumen, (the whites of eggs,) which thus became a by no means unimportant article of trade. It is used to condense, and to aid in spreading, the color, but requires some slight admixture of oil to prevent the decomposition which the albumen, pure and simple, was found to produce. Ultramarine is used not only to produce a blue, but also a white. Every housewife well knows that blue of some kind must be used to counteract that yellowish tinge which linen and cotton goods acquire when washed. This use of the blue color is familiarly called using the blue-bag, but using the whitening bag would, in truth, be the more appropriate phrase. As a general thing the blue-bag is used far too freely. The effect should not be, as it generally is, to leave a blue tinge, but only to neutralize that yellow tinge with which we unavoidably associate the idea of imperfect cleansing. Ultramarine is also of important service in restoring linen and cotton yarns and fabrics to good color— from two to three pounds of the color sufficing to restore fifty pieces of linen. From ten to fifteen ounces are sufficient for the perfect bleaching of twenty pounds of yarn, and so effective is it in small quantities, and therefore so cheap, that even whitewashers use it to give increased brightness and cleanness to their white. It was formerly considered, on toxicological grounds, that the use of ultramarine in whitening sugar was objectionable. We need here only so far advert to the discussions of the public journals upon that point as to say that two pounds and a half of ultramarine suffice to bleach fifty tons of sugar, being just of grain to the pound, a proportion in which even that deadly corrosive, arsenic, would be entirely innocuous. Whether the sulphuric-hydrogen gas, which is liberated by the contact of the ultramarine in the sugar with the acid in wine, be offensive, is a question which we leave to the olfactories of the chemist to decide. How far ultramarine is, or may be, adulterated, chemists, we believe, have not, as yet, determined. Manufacturers maintain that it is not merely right, but even necessary, to mix potter's clay and gypsum with ultramarine, in order to get a lighter color; and to us it seems that, on that point at least, the manufacturer is a better judge than the chemist. The purchaser well knows that such admixture is made, and for what purpose, so that, whether right or wrong, there is, at all events, no deception; but if he wishes no such admixture in the ultramarine which he purchases, a simple and facile test of the quality is at hand. Adulteration is present if the color be not entirely discharged by strong acids, or if it change color when boiled in a ley of potash. The adulteration in this latter case has been made by organic matters, for the purpose of producing the fiery brilliancy of the natural ultramarine. If, to be thus tested, the ley assume a greenish tinge, the ultramarine contains a superfluous amount of sulphur natrium; and if the ultramarine adheres in hard clots or lumps the salts have not been sufficiently washed from it. When mere apearance is alone relied on as a criterion, the judgment, however practiced. is iable to be mistaken, for there is no other color which affords so much scope for visual deception. There are two qualities to be regarded in the genuine ultramarine—the coloring and the covering quality—which maintain no direct ratio one to the other. The coloring quality may be tested by mixing one part of ultramarine with ten parts of any white color—white lead, for instance, or clay, or gypsum—and then closely observing the tone of the mixture. These trials should never be omitted by purchasers, for in two ultramarines, which to the sight appear exactly alike, there may be a difference, in both brilliancy and durability, of from one to two hundred per cent. Another important question is this: How much mordant does the particular ultramarine require Nor is this important only in those great factories where the mordants are a considerable item of expense, for the artist also should be aware that every addition of mordant diminishes the clearness of the color. The less mordant the finer color, and vice otersa. It admits of no doubt that from remote antiquity the art of coloring of the raiment with which man invested himself ...} acquired a certain degree of proficiency. Pliny, though he gives no particulars of the processes, yet assures as that the ancients were well £o with the use of mordants, by which fixity is given to colors which otherwise would gradually change by successive gradations, or disappear altogether from the dyed fabric. Of those mordants he mentions human urine, ammonia, and certain salts, including rock-salt and soda, as serving to give at once brilliancy and fixity of color to spun and woven stuffs. And in another passage he intimates a still more advanced knowledge of the art of dyeing as practiced by the ancients. “In Egypt,” he says, “ cloths are dyed in a quite peculiar manner. The cloth is first thoroughly cleansed and then successively dipped into one or more solutions, and finally into the fluid color for which the previously used solution has so great an affinity that the cloth is dyed as permanently as instantaneously. What is most remarkable about this process is the fact, that though the dye-vat contains dye of only one color, the web of cloth is dyed of one, two, or several colors, according to the kind of solutions used for the preliminary washings or dippings. And further, not only is the cloth so permanently dyed that the color cannot be washed out, but the cloth itself is rendered stronger and more durable.” This language of Pliny shows, that our knowledge of the uses and effects of various mordants to heighten and fix color, and rather to improve than to injure the fabric of the stuff to be dyed, though doubtless much indebted to modern chemistry, is, substantially, as old as chemistry itself. In the case of ancient Egypt, such a knowledge need scarcely excite our surprise, that antique and mysterious land having been the source of the chemical science of at least all the people of antiquity. As nature herself suggested colored ornamentation, and the fugitive qualities of the earlier dyestuffs forced chemistry into the discovery of mordants, so the lack of a cultivated taste made the glaring scarlet and tawdry yellow the favorites of the earlier ages; just as, in our day, the same lack or imperfection of taste is apt to recommend those vivid hues to the favor of the childish and the unrefined. Next to the Egyptians the people of ancient India evinced most skill in the art of coloring. Job speaks with great admiration of the brilliant colors of Indian cloths. There is at this day in the museum of the Industrial Society at Paris a large and valuable collection of Indian colored stuffs, together with the utensils by which they were prepared. These stuffs should be called painted rather than dyed; the absorbent and mordant fluids were first applied with a brush, and the desired colors then laid on; those portions which were to remain white were at the outset covered with wax, and the outlines of the pattern traced on the remainder. There is also at Paris a shawl, ten feet long and five feet wide, the handiwork of Indian princesses, and so elaborately as well as beautifully executed that it must have employed the skill and industry of more than one generation of the royal and dusky workwomen. But everything else in ancient dyeing was surpassed by the proverbially pre-eminent

TYRIAN PURPLe.

Inventions have their place in Mythology, and not improperly; for if chance plays no inconsiderable part in the inventions and discoveries of the present ". so, also, it did in the days of old. All have heard, or read, the story of the dog

« AnteriorContinuar »